Data Types

Lists

List is a sequential data type. List values are comma separated and enclose with square bracket.
List in Python is like array in Perl and an array(ArrayList Class) in Java.

>>>

>>> tmp_list = ["a", "b", "z"] # Defining Lists

>>> tmp_list

['a', 'b', 'z']

>>> tmp_list [0] # accessing element of list

'a'

>>> li[2]

'z'

>>> tmp_list[4] # try to read or write an element that does not exist, gives an IndexError

Traceback (most recent call last):

File "<pyshell#1>", line 1, in <module>

tmp_list[4]

IndexError: list index out of range

>>> dir(tmp_list) # dir will list-out all attributes of object

['__add__', '__class__', '__contains__', '__delattr__', '__delitem__', '__delslice__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__getslice__', '__gt__', '__hash__', '__iadd__', '__imul__', '__init__', '__iter__', '__le__', '__len__', '__lt__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__reversed__', '__rmul__', '__setattr__', '__setitem__', '__setslice__', '__sizeof__', '__str__', '__subclasshook__', 'append', 'count', 'extend', 'index', 'insert', 'pop', 'remove', 'reverse', 'sort']

Key features of List:

List items can be any datatype, within a single list you can have multiple datatype as item
List can have duplicate values
List is Mutable object
Negative index access value from end of the list i.e. tmp_list[n-1] = tmp_list[-1]
You can use any integer expression for index

Adding items to list, you can insert item to any place in the list using insert method. append method will insert item at the end.

>>>

>>> li = ['a', 'b', 'c']

>>> li.append('d')

>>> li

['a', 'b', 'c', 'd']

>>> li.insert(1,'x')

>>> li

['a', 'x', 'b', 'c']

>>> li.insert(-1,'y')

>>> li

['a', 'x', 'b', 'y', 'c']

>>> li.append([1,2]) # you can add list as an item

>>> li

['a', 'x', 'b', 'y', 'c', [1, 2]]

List has extend and append method,both add values to list but both work differently.
Extend takes single argument,which is always list whereas append can n number of argument and add each item to original list.

Category: Data Types, Lists | Comments Off

September 24

Dictionary

Dictionary is a key value pair,mapping between a set of keys and a set of values.
Association of a key and a value is called key-value pair or an item.
Python dictionary is like hash in Perl, an instance of Hashtable class in Java or an instance of the Scripting.Dictionary object in Visual Basic.

>>> tmp_dict = {"key1":"value1", "key2":"value2"} # create a new dictionary with two elements and assign it to the variable tmp_dict

>>> dmp_dict # Key1 is a key and its associated value value1

{'key1': 'value1', 'key2': 'value2'}

>>> tmp_dict["key1"] # get access by key

'value1'

>>> tmp_dict["key2"]

'value2'

>>> tmp_dict["key3"] # If key is not there then you get KeyError

Traceback (innermost last):

File "", line 1, in ?

keyError: key3

>>> tmp_dict["key1"]="value3" # modifying value of dictionary

>>> dmp_dict

{'key1': 'value3', 'key2': 'value2'}

>>> dir(tmp_dict) # dir give list of all attributes of object

['__class__', '__cmp__', '__contains__', '__delattr__', '__delitem__', '__doc__', '__eq__',

'__format__', '__ge__', '__getattribute__', '__getitem__', '__gt__', '__hash__', '__init__',

'__iter__', '__le__', '__len__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__',

'__repr__', '__setattr__', '__setitem__', '__sizeof__', '__str__', '__subclasshook__', 'clear',

'copy', 'fromkeys', 'get', 'has_key', 'items', 'iteritems', 'iterkeys', 'itervalues', 'keys',

'pop', 'popitem', 'setdefault', 'update', 'values', 'viewitems', 'viewkeys', 'viewvalues']

>>>

>>> help(tmp_dict) # help generates its document as text on console

Help on built-in function iteritems:

iteritems(...)

D.iteritems() -> an iterator over the (key, value) items of D

>>>

>>> tmp_dict.keys() # It will give you all the keys in list format

['key1','key2']

>>> tmp_dict.values() # It will give you all the values in list format

['value3','value2']

Key features of dictionaries

dictionary can not have duplicate keys
assigning new value to existing key will simply replace the old value
dictionary value can be any datatype, within a single dictionary you can have multiple datatype as value

>>>

>>> tmp_dict = {"key1":"value1", "key2":2,"key3":[1,2],"key4":(1,2),"key5":{"key6":"value6"}}

>>> tmp_dict

{'key3': [1, 2], 'key2': 2, 'key1': 'value1', 'key5': {'key6': 'value6'}, 'key4': (1, 2)}

>>>

dictionary key can be any immutable datatype

Deleting Items From Dictionaries
Dictionary has two method to delete/clear the data

del, will delete individual item from dictionary
clear , will clear all the data and give you a empty dictionary

>>>

>>> tmp_dict

{'key3': 3, 'key2': 2, 'key1': 1}

>>> del tmp_dict['key2']

>>> tmp_dict

{'key3': 3, 'key1': 1}

>>> tmp_dict.clear()

>>> tmp_dict

{}

Category: Data Types, Dictionary | Comments Off

September 21

Python provides built-in data-types like Dictionary, Lists, Tuples.

Category: Data Types | Comments Off